Speed and sound freezing issue

For a reason I do not know, my Mac started to act strangely lately. It slow down significantly and the music sound would stop playing and start again the mouse icon turns much more than usual.
I checked the hard disk all seems ok 84 Giga still available and no faults.
Today I added one more Giga of memory but still I have my I tune which stops and start playing again, I could hear the disk swapping like f its searching for something-.

What could cause that, I just did the upgrade recommended by apple did not load anything new.

Thank you for your help

Overview:

Model Name: iMac
Model Identifier: iMac5,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.16 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 4 MB
Memory: 3 GB
Bus Speed: 667 MHz
Boot ROM Version: IM51.0090.B09
SMC Version (system): 1.9f4